The motion of test particles and cosmological interpretations: the role of MOND

Throughout history, observations of the motions of objects in the Universe have provided the foundation for various cosmological models. In many cases, the invoked causes of the observed motion appeal to mysterious elements. Indeed, the very first test motion was that of the retrograde motion of Mars, which lead to a required epicycle to save the model (e.g., Ptolemy's unmoving Earth). By the early 1840s, from approximately 50 years of orbital data (since its 1789 discovery) it was apparent that Uranus was disobeying the Newtonian rules in its orbit and speculation mounted that a "large unseen mass" was perturbing the orbit. Using Uranus as a test particle then yields the first notion of dark matter (DM). Alas, it was not DM but merely Neptune, discovered in September 1846. By 1859 enough data had been gathered to reveal that Mercury is also not obeying Newtonian physics but rather revealing curved space-time. The continuation of this history is now set in scales larger than the Solar System. Observations suggest two basic choices: (i) gravity is fully understood and Newton's second law is invariant (except in very strong gravity) and observed motions on galactic scales require the existence of DM (a currently unproven "epicycle") or (ii) Newton's second law can be modified (e.g., MOND) in certain low acceleration scale environments. In this contribution we discuss the case for and against MOND on various scales and conclude that neither MOND nor our current cosmology (Lambda CDM) consistently explain all observed phenomena. In general, MOND works much better on small scales than Lambda CDM but encounters difficulties on large scales. Moreover, the nature of the acoustic power spectrum of the CMB now pretty clearly shows that a fully baryonic Universe is ruled out, thus necessitating some DM component. But this should not diminish the consideration of MOND as its introduced acceleration scale; a(o) is fully consistent with the observed structural properties of galaxies in a way that the DM halo paradigm cannot match. Indeed, despite many attempts to falsify MOND, it has always come back from its proclaimed death to provide unique insights into the gravitational nature of galaxies, consistently raising the specter that our current understanding of gravity acting over large spatial scales may be flawed.